The present invention relates to medical engineering. More particularly the invention relates to a method of indirect measurement of arterial tension and a device for pulse wave registration.
The invention will find most widespread application in indirect determination of patient's arterial tension at an increased ambient noise level, and at an increased level of noise produced by patient's motions.
One prior-art method of measuring arterial tension with the aid of a compression cuff and a sensor of the Korotkov sounds (cf. "Acoustic method of arterial tension examination" by G. I. Kositsky, Medgiz PH, Moscow 1959, pp. 53-54 (in Russian) includes the application of a compression cuff to the surface of the patient's body. The pressure in the cuff is varied within a range covering the systolic and diastolic values of the patient's arterial tension and the Korotkov sounds produced by the arterial pulse waves issuing from under the cuff are simultaneously registered via a microphone, said values corresponding to the instants when the Korotkov sounds appear and disappear.
However, the aforesaid method is disadvantageous because the microphone picks up not only the Korotkov sounds, but also ambient noise, and in some cases it fails to measure the arterial tension value when the measurement process is automated, since the loudness of the Korotkov sounds, their spectral composition and some other characteristics are substantially different in every patient.
A more perfect method of measuring the arterial tension, capable of more accurate determination of the extreme values of such tension is a known method, which is based upon registration of a pulse wave and is insensitive to ambient noise. This method includes the application of a compression cuff to the surface of the patient's body. The pressure in the cuff is varied within a range which covers the systolic and diastolic values of arterial tension and pulse waves resulting from the releasing of the artery compressed by the cuff are registered. The pressure in the cuff is measured at the instants when these pulse waves appear and disappear. The pulse waves are registered under the cuff on the body surface area located opposite the artery and beyond the midpoint of the cuff as along the direction of the arterial blood flow, against displacement of the surface of the patient's body in a direction square with that surface (cf. "Automatic meter for indirect measurement of arterial tension" by V. M. Bolshov et al., "Medtekhnika", No. 2, 1979, pp. 19-22 (in Russian).
However, the aforedescribed method is disadvantageous in that the accuracy of measurement of the systolic and diastolic arterial pressures is too low. This disadvantage stems from the fact that the aforesaid displacement of the patient's body surface in a direction square with that surface may occur without any pulse waves passing under the cuff, that is, without releasing the artery, but may result from pressure fluctuations in the cuff which are liable to arise usually before releasing the compressed artery, or from muscular contractions of the body portion located under the cuff, or else it may be caused by elastic deformation of the walls of the filled artery when the pressure in the cuff is below the diastolic value.
It becomes evident from the diagrams obtained from determining the arterial tension by the known method that the signals resulting from the noise, e.g., due to the patient's muscular contractions have an amplitude much greater than that of the useful signals produced by the blood pulse waves. This results in that a signal is registered from the noise alone, without any pulse wave passed, which in turn leads to incorrect readings of the arterial tension values.
The heretofore-known device for registering blood pulse waves in indirect measurement of arterial tension (cf. USSR Inventor's Certificate Ser. No. 651,786, class A61B 5/02 dated 1979) comprises a housing and a transducer rigidly secured to the housing, said transducer being adapted to convert mechanical motions into electric signals. A pellet is secured to the transducer traversable portion, said pellet having a contact surface and being adapted to impart to said transducer the displacements of the patient's body surface square with said surface and hence with the pellet contact area.
The transducer is in effect a disk-shaped piezocrystal and the pellet, also disk-shaped and made from a hard material such as plastic, located at the central axis of the transducer.
The known device is arranged under the compression cuff past its midpoint in the direction of the blood flow.
However, the aforedescribed device fails to provide high-accuracy measurement of the systolic and diastolic values of arterial tension, since it is sensitive to pressure oscillations in the cuff and to motions of the body surface due to muscular contractions. Thus for instance, when measuring arterial tension under decompression conditions, pressure oscillations arise in the cuff before a first pulse wave passes under the cuff, from which the systolic value of the arterial tension is measured. These pressure oscillations create a force which is exerted on the housing of the device to cause mutually opposite displacements of the housing and pellet. This results in a false signal appearing at the output indicating the passing of a pulse wave, and in a mismeasured systolic value of the arterial tension. The pellet is displaced in the same manner due to muscular contractions under the device occurring at a pressure in the cuff below the diastolic value and due to an elastic pulsing deformation of the walls of filled artery with the resultant false signals at the output of the known device. All this leads to inaccurate determination of the systolic and diastolic values of the arterial tension and renders it impossible to measure arterial tension with the patient moving.